1. Field of the Invention
This invention relates to digital transmission equipment and more particularly to digital transmission equipment connected to a 2-wire subscriber line and having a local loopback testing function.
2. Description of the Related Art
According to high speed and digitalization in transmission network systems, digital equipment that sends and receives digital signals has been more popularly installed not only in main routes of the system but also at subscribers.
This situation has required testing methods for finding out failure positions in the system to receive transmitted signals with fewer faults.
A local loopback testing is one of the testing methods. FIGS. 1(A) and 1(B) illustrate a general concept of the local loopback testing in a digital transmission system. In FIGS. 1(A) and 1(B), the reference numeral 1 is network termination equipment (NTE). The reference numeral 2 is exchange or switch equipment (EX). The reference numeral 3 is subscriber equipment or terminal equipment (TE), and the reference numeral 21 is line termination equipment (LTE) that is provided in the exchange equipment 2.
The line termination equipment 1 and the network termination equipment 21 are also referred as digital termination equipment, because they have the same function as explained later.
The reference numeral 4 is a 2-wire subscriber line. For example, digital signals of a 4-level pulse amplitude are transmitted over the 2-wire subscriber line between the exchange equipment 2 and the network termination equipment 1. The digital signals of a 4-level pulse amplitude are defined as 2B1Q code signals by the T1, 601 ANSI North American 1991 Standard. The numeral 5 is two 2-wire lines, which connect the network termination equipment 1 and the terminal equipment 3.
In FIG. 1(A), a loopback testing is executed to confirm the operation of the network termination equipment 1, and FIG. 1(B) explains a loopback testing for the line termination equipment 21. Sending signals are loop-backed in the digital termination equipment 1 and 21 as shown respectively in FIG. 1(A) and FIG. 1(B), when the loopback testing is executed. The defect or failure of operations in the equipment 1 and 21 is checked by detecting the loop-backed signals.
The local loopback testing will be explained in more detail referring to FIGS. 2(A) through 2(D). Although FIG. 2(A) illustrates a part of the structure of the network termination equipment 1, the line termination equipment 21 has the same structure and function as those of the network termination equipment 1.
The part of the structure of the network termination equipment 1 is composed of a sending circuit 101, an interface transformer 14, a receiving circuit 102 and an echo canceler circuit 103. A pair of terminals T and R are connected to the 2-wire subscriber line 4.
In a transmission mode, the sending circuit 101 groups sending digital signals (a) into pairs of bits and codes the pairs of bits to digital signals (a') of a 4-level pulse amplitude. The coded digital signals are sent out to the 2-wire subscriber line 4 through the interface transformer 14.
On the other hand, the receiving circuit 102 receives through the interface transformer 14 digital signals (b) of a 4-level pulse amplitude sent from the line termination equipment 21 that is provided in the exchange equipment 2. The digital signals received by the receiving circuit 102 are decoded into digital signals (c) and the decoded digital signals are then sent to the echo canceler 103.
The echo canceler 103 subtracts an echo-backed portion (a) of the sending digital signals from the digital signals received and decoded by the receiving circuit 102.
The digital signal trains illustrated in the references (a) through (d) of FIG. 2(B) show signals at the corresponding positions (a) through (d) of the network termination equipment 1. In FIG. 2(B), the pulse train of digits such as (0, 1, 0, 1, 0, 1, 1, 0) with the reference (a) is the output of the sending circuit 101.
The pulse train of digits (0, 1, 1, 0, 0, 1, 1, 0) at the reference (b) corresponds to digital signals transmitted from the line termination equipment 21 and appears at the position (b) in FIG. 2(A). The signals at the reference (b) should be illustrated as digital signals of a 4-level pulse amplitude as explained above, but they are illustrated in the form of decoded digital signals to simplify the illustration of FIG. 2(B).
The reference (c) shows the pulse train of digits (0, 2, 1, 1, 0, 2, 2, 0), which is composed of the signals transmitted from the line termination equipment 21 at the reference (b) and a portion, of the sending signals, echo-backed through the interface transformer 14 at the reference (a).
The reference (d) shows the pulse train at the output of the echo canceler 103, which is the same as that of the references (b), because the pulse train of the reference (a) is subtracted by the echo canceler 103 from the pulse train of the reference (c).
As explained above, the role of the echo canceler 103 is to eliminate an echo-backed portion of the sending digital signals from digital signals received from the line termination equipment 21. Then, digital signals may be received at the network termination equipment 1 without error or with less error rate.
FIGS. 2(C) and 2(D) show examples of signals at the corresponding positions (a) through (d) of the network termination equipment 1 on loopback testing modes. FIG. 2(C) shows the pulse trains of the digital signals, where the digital signals of a 4-level pulse amplitude transmitted through the 2-wire subscriber line 4 are blocked off at the position (b), and the echo canceler 103 is turned off. Therefore, the output digital signals from the echo canceler 103 are the same as the sending digital signals at the position (a), if the circuit blocks shown in FIG. 2(A) are in a normal operation or right condition.
FIG. 2(D) shows the pulse trains, where digital signals transmitted through the 2-wire subscriber line 4 are blocked off at the position (b) and the operation of the echo canceler 103 is still active.
In this case, the echo canceler 103 subtracts sending digital signals from the echo-backed signals and therefore, the output of the echo canceler 103 becomes all "0" signals as shown in the reference (d) of FIG. 2(D) if the circuit blocks of FIG. 2(A) are in normal state. Thus, it can be determined whether the operation of the circuits shown in FIG. 2(A) is in normal state or not, considering the output from the echo canceler 103.
From the foregoing explanation, it should be understood that the blocking off or disconnection at the position (b) between the interface transformer 14 and the 2-wire subscriber line 4 is necessary when a local loopback testing is executed.
FIG. 3 illustrates a conventional structure of the network termination equipment 1. In FIG. 3, the reference numeral 10 is an interface transceiver 10, in which there are provided the sending circuit 101, the receiving circuit 102 and the echo canceler 103 such as explained concerning FIG. 2(A).
The interface transceiver 10 may be constructed in an IC chip. The IC chip as coded T7264, a product of AT&T can be used for the interface transceiver 10. The reference numeral 11 is an interface transformer comprising two transformers T1 and T2.
The first windings of the transformers T1 and T2 are connected to the terminal equipment 3 through a pair of 2-wire subscriber lines 5 (shown in FIGS. 1(A) and 1(B) and the second windings are connected to a line termination block 12.
The line termination block 12 has the role of supplying a test pattern signal generated in a test pattern generator 121 to the interface transceiver 10 at the local loopback testing. The line termination block 12 also extracts a loop-backed test pattern signal and sends it to a test pattern detection circuit 122.
The test pattern detection circuit 122 detects whether the loop-backed test pattern signal is the same as that generated in the test pattern generator 121 or not. Then, the test pattern detection circuit 122 concludes if the function of the circuits through which the test pattern signal flows is in normal state or not.
The reference numeral 13 is a 2/4-wire converter that converts 4-wire mode signals supplied from the line termination block 12 to 2-wire mode signals and in reverse 2-wire mode signals supplied from the interface transceiver 10 to 4-wire mode signals.
The 2/4-wire converter 13 also includes a loopback instruction detector 110. The detector 110 is to detect, for example, a predetermined code pattern signal sent from the terminal equipment 3 or a switch information given by a manually operable switch which is provided in the network terminal equipment 1, but is not shown in the drawing.
The reference numeral 14 is an interface transformer, which transfers signals between the interface transceiver 10 and a 2-wire subscriber line 4 (shown in FIGS. 1(A) and 1(B), isolating dc currents.
The reference numeral 15 is a loopback relay block comprising a pair of mechanical relays RL1 and RL2 positioned on signal transmission paths as shown in FIG. 3.
The mechanical relays RL1 and RL2 are driven by the loopback testing instruction signal generated in the loopback instruction detector 110 to make the signal transmission paths open. This prevents digital signals of a 4-level pulse amplitude from flowing to the 2-wire subscriber line 4 and in reverse to the network termination equipment 1 at the local loopback testing.
However, the conventional structure as illustrated in FIG. 3 employs mechanical relays RL1 and RL2. Therefore, there has been the possibility such that digital signals of a 4-level pulse amplitude cannot flow between the network termination equipment 1 and the 2-wire subscriber line 4 in a normal signal transmission mode. The possibility is caused by the disconnection of contacts of the mechanical relays RL1 and RL2.
Such the disconnection may occur due to physical noises or vibrations given from outside the equipment. Consequently, the reliability of digital signal transmission should be diminished in a conventional transmission system.
To avoid such problems as existing in the conventional equipment, there has been proposed alternative ways. In one of the alternative ways, an operator physically disconnects the 2-wire subscriber line 4 from the network termination equipment 1 at the loopback testing in stead of providing mechanical relays on signal transmission paths.
However, such a physical disconnection has not been practical and cumbersome for the operator. Therefore, as second alternative way, a loopback point is provided at digital circuits in the digital terminal equipment 1 for a local loopback testing, if even such the physical disconnection is not possible.
But in such the case, portions external to the digital circuits of the equipment are not subject to a local loopback testing.
Therefore, such the testing by providing a loopback point at the digital circuits cannot accord with the purpose of the local loopback testing to detect any failure portion in a digital network system and to disconnect the failure portion from the system.